Accordion

Collapsible sections with real headings and a link between button and panel.

The heading is a real `<h3>` with the button inside it, not a button that
looks like a heading. That is what lets a screen reader jump between sections.
Bits renders `<div role="heading">`, which is correct for a screen reader —
we swap it for the element, because an element also holds where ARIA does not:
reader modes, printing, tools that read structure without running JavaScript.

**Bits does not set `aria-controls` on accordions**, although it does on tabs.
That is a gap against WAI-ARIA's own pattern, so the item generates the id
itself and connects both ways. This is exactly what the third layer is for:
Bits owns the behaviour, we own that it is complete.